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Foam Drainage: Microscale Flow in an Ideal Isolated System C. Clarke*, A. Lazidis, F. Spyropoulos, I.T. Norton *Department of Chemical Engineering, The University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK ̴120˚ FLOW Drainage The driving force for foam collapse Aims • Quantify how food grade surfactants and particles impact liquid flow through foams on a microscopic scale • Scale up to see how this impacts the drainage of liquid food foams on a macroscopic scale • Provide a visual technique to identify microscopic flow characteristics (e.g. velocity, geometry, resistance), potentially leading to more tailored foam control Drainage Channels – Plateau borders & Nodes • Fluid flow occurs through a network of channels which take on a distinctive geometric shape as the liquid fraction decreases • Each channel made up of the intersection of three films, and terminates at a node with three additional channels Plateau border Node Issues with Existing Work • Existing experimental work looks at microscopic flow through ‘infinite’ Plateau borders that do not terminate in nodes[1][2] • The only study (to this author’s knowledge) that observes flow velocity through an isolated Plateau border and node uses a warped node shape not described by theory[3] • No exploration of changes to the shape of the border and node for food grade formulations at typical flow rates • Conflicting theoretical models describing resistance of nodes to liquid flow 1 Koehler, S. A., et al. (2004). "Foam drainage on the microscale II. Imaging flow through single Plateau borders." Journal of Colloid and Interface Science 276(2): 439-449. Pitois, O., et al. (2005). "Liquid drainage through aqueous foam: study of the flow on the bubble scale." Journal of Colloid and Interface Science 282(2): 458-465. 3 Pitois, O., et al. (2008). "Node contribution to the permeability of liquid foams." Journal of Colloid and Interface Science 322(2): 675-677. 2 Why do nodes matter? • Two main flow regimes exist in foams: Plateau Border Dominated: Main flow impact from shape and surface rheology of the Plateau border Node Dominated: Main flow impact from shape, surface rheology and any extra material blocking the node • Probing both regimes may reveal characteristics that help to clarify best choice of ingredients for foaming solutions Some key methods of analysing foam drainage… • Observation • Electrical Conductance • Only shows average liquid content with time • Tomographic Imaging • Even fasted acquisition times struggle due to foam instability • Confocal Microscopy • Limited depth of imaging and requires high optical clarity • MRI • Labelling components may influence their actions Isolated Plateau Border Setup • 3D printed setup adapted from the work of Pitois et. al.[1] to produce an ideal Plateau border and node geometry Connection to Syringe Pump for controlled flow rate injection of foaming solution Precision injection nozzle determining flow velocity at entrance to Plateau Border Angled ‘tripod’ arrangement to ensuring stationary attachment of films at 120˚ to one another Adjustable base to define length of Plateau border 1 Pitois, O., et al. (2008). "Node contribution to the permeability of liquid foams." Journal of Colloid and Interface Science 322(2): 675-677. Basic Imaging Using Setup • Photron SA3 high speed camera • White light illumination • Image scale using syringe needle Basic Imaging • PB and Node shadow images • 510μm diameter needle for permanent scaling feature • Interference fringes in films showing localised thinning Measuring PB Radius • Plot horizontal intensity profiles at vertical intervals • Apply geometric correction factor of: 2 1 Elias, 3 [1] F., et al. (2014). "Elasticity of a soap film junction." Physics of Fluids 26(3): 037101. Plateau Border Radius (μm) 0 TOP 100 200 300 400 500 600 700 800 Distance from Outlet (μm) 0 5000 10000 15000 0.35% Fairy Liquid 0.7% Fairy Liquid 20000 25000 BOTTOM 30000 PB Radius vs Distance from 0.4mm diameter Outlet for 0.35% and 0.7% Fairy liquid solutions at 0μl/min 900 Initial Results • Testing the performance of the setup and beginning to compare to theoretical models for PB geometry and node resistance at varying flow rates • Solutions of 0.7% and 0.35% Fairy Original washing up liquid with 7.5% glycerol are currently used for their high foamability and stability • Flow rates from 0μl/min to 350μl/min are used (from free drainage to forced drainage regimes) Expansion Pinch Choosing different surfactants to influence flow • Different stabilisation mechanisms of surfactants impact the flow Proteins LMWS Surfactants • Rigid boundaries Large flow increase towards channel centre Lower average flow velocity • Mobile boundaries More uniform flow between centre and edges Higher average flow velocity Mobile Nguyen, A. V. (2002). "Liquid Drainage in Single Plateau Borders of Foam." Journal of Colloid and Interface Science 249(1): 194-199. Rigid Ongoing… • Use of proteins and other food grade surfactants (e.g. egg white proteins, tweens…) • Tests to assess the impact of blocking the node on the flow (e.g. particles) • Quantify node resistance and average flow velocity using pressure measurements • Visualise flow with fluorescent particle/dye tracking Koehler, S. A., et al. (2004). "Foam drainage on the microscale II. Imaging flow through single Plateau borders." Journal of Colloid and Interface Science 276(2): 439-449. Any Questions?